1. Field
The present innovations relate generally to methods and systems associated with locating events in real time using sensor arrays. More specifically, it relates to techniques for improving the performance of weapon fire location systems using data from an acoustic survey.
2. Description of Related Information
An urban gunshot location system has to detect gunfire in a complex and noisy acoustic environment. Because of the plethora of sounds present, a method is needed to discard the majority of non-explosive sounds and concentrate on sounds that can eventually be classified as gunfire. That problem was addressed in U.S. Pat. No. 5,973,998 “Automatic Real-Time Gunshot Locator and Display System.” A key disclosure therein is the “Spatial Filter” concept, wherein widely-spaced sensors can only triangulate on sounds loud enough to traverse the large distances between several sensors.
A useful corollary to the spatial filter is that if a 4th sensor has an impulse at the time expected from the initial triangularization result, the location is robust enough (i.e., unlikely to be from unrelated sounds) so that it can be automatically presented to dispatchers. Multiple gunshots detected at only 3 sensors allow robust detection and hence dispatch from those 3 sensors alone, as they are nearly certainly correlated.
In an urban setting the buildings cause refraction, reflection, and blockage of audio waves propagating from gunfire to the various sensors. In moderately built up communities with a majority of 2 and 3 story buildings, the errors due to refracting around buildings can be 15 to 30 feet (in a flat area of a mile extent, the error will only be 10 feet). Reflections (multipath) are usually not important to triangulation if the direct path is also received, as subsequent impulses can be discarded. But in practice, one cannot be certain that the first impulse is a direct path due to the possibility of blockage. Presently the existence of a hole in the coverage area can only be determined if a known shot is not detected. Also, the sensitivity of the array at a given location to a given sized weapon can only be guessed instead of determined by measurement.
An attempt to mitigate against blockage is contained in U.S. patent application Ser. No. 11/546,529, filed 10 Oct. 2006, entitled “Acoustic location of Gunshots Using Combined Angle of Arrival and Time of Arrival Measurements,” which is incorporated herein by reference. In this application the sensors determine the arrival times and the azimuths of the arriving impulses. So if the location determined from a given triad of sensors is consistent with the azimuthal measurements, then that triad can be trusted for this event. If the sensor azimuths do not point to the location then that sensor or the triad cannot be trusted. Azimuthal sensors also allow a fewer number of sensors to locate and confirm a location, and they can lift mathematical ambiguities for solutions where two roots are both acceptable.